Reducing supply to ground current

ABSTRACT

An apparatus to prevent supply-to-ground current in a comparator is disclosed. The apparatus includes circuitry to determine if first and second output nodes of the comparator have respectively reached first and second logic levels, and circuitry responsive to a determination that the voltage at the first and second output nodes of the comparator has reached the first and second logic levels, to generate a signal. In addition, the apparatus includes circuitry to supply the signal to a transistor, the signal to turn off the transistor and prevent the flow of supply-to-ground current through the comparator.

FIELD OF THE DISCLOSURE

This disclosure relates generally to supply-to-ground current, and, moreparticularly, to the reduction of supply-to-ground current.

BACKGROUND

In electronics, a comparator is a device that compares two voltages orcurrents and outputs a digital signal indicating which is larger. Acomparator typically includes two analog input terminals V+ and V− andone binary digital output V. It is commonly used in devices that measureand digitize analog signals, such as analog-to-digital converters.Comparators receive an analog signal and produce a digital bit.Comparators are evaluated based upon their ability to distinguishbetween tiny differences in input voltages as quickly as possible. Inorder to reduce the effect of input referred offset voltage, apreamplifier and latch circuit architecture is commonly used to achievehigh speed and accuracy. Challenges to the optimal performance ofcomparators include the attenuation of accuracy due to offset voltage,the effect of kickback noise on resolution and excessive currentconsumption.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is diagram of a comparator that uses a preamplifier and latcharchitecture as described in detail with reference to FIG. 2A.

FIG. 2A is a schematic of an example implementation of the comparatorcircuit of FIG. 1 that includes a differential amplifier and latcharchitecture.

FIG. 2B is a graph of the through current that is conducted in thecomparator of FIG. 2A.

FIG. 3 shows an example comparator that includes a differentialamplifier and latch circuit and a supply-to-ground current preventionsignal generator.

FIG. 4 shows a first example implementation of the supply-to-groundcurrent prevention signal generator shown in FIG. 3.

FIG. 5 shows another example of a supply-to-ground current preventionsignal generator.

FIG. 6 shows an example implementation of FIG. 5.

FIG. 7 shows an example implementation of FIG. 5.

FIG. 8 shows components of a supply-to-ground current prevention signalgenerator.

FIG. 9 shows a method to generate a supply-to-ground current preventionsignal.

FIG. 10 is a block diagram of an example processor platform capable ofexecuting the instructions of FIG. 9.

The figures are not to scale. Wherever possible, the same referencenumbers will be used throughout the drawing(s) and accompanying writtendescription to refer to the same or like parts.

DETAILED DESCRIPTION

The comparator in a Sigma Delta analog-to-digital converter (ADC) runsat a sampling rate. The time available to resolve input voltagedifferences at high sampling rates is very short. In order to resolveinput voltage differences at higher sampling rates, the use of a veryhigh power comparator is involved.

In an example technology, flicker noise in PMOS-type architectures isabout 10 times lower than in NMOS-type architectures. Because meetingnoise specifications for noise critical blocks in the ADC is important,a PMOS-based architecture can be used for noise critical blocks (like adigital-to-analog converter-DAC, gmCell). In addition, PMOS-basedarchitectures simplify the biasing needs of the example ADC. PMOS-basedcomparators can be slower than NMOS-based comparators for the same sizedtransistors. Examples described herein provide a high speed and lowpower PMOS-based comparator.

FIG. 1 is a comparator 100 that uses a preamplifier and latcharchitecture. Referring to FIG. 1, the preamplifier 101 includes inputsINn 103, and INp 105, and clock 107. The latch 109 includes outputs outn111, and outp 113, and clock 115. The architecture of FIG. 1 enables thesize of the latch 109 to be reduced because of the use of thepreamplifier 101. For example, in ADC with Fs=1.6 GHz the preamplifier101 and the latch 109 use 640 uA (e.g., preamplifier 470 uA and latch170 uA).

FIG. 2A is a schematic of an example implementation of comparatorcircuit 100 of FIG. 1 that includes a differential amplifier and latcharchitecture. FIG. 2A shows a differential amplifier 200 that includestransistor 202 and transistor 203, input INP 204, transistor 205, inputINM 206 and outputs OUTM 209 and OUTP 211. The differential amplifier200 is coupled to a latch 212 that includes transistors 213, 215, 229and 231.

Referring to FIG. 2A, inputs INP 204 and INM 206 to the differentialamplifier 200 are received by the transistors MP0 203 and MP1 205. Insome examples, when the CLK 201 goes “low,” the PMOS transistors (MP0203, MP1 205, MP2 213, MP3 215) are switched ON. In response, adifferential voltage is developed at nodes OUTM 209 and OUTP 211.Moreover, by virtue of the operation of the latch 212, the nodes OUTM209 and OUTP 211 go to either supply or ground (e.g., the voltage at oneof the nodes goes to supply or logic 1 and the other of the nodes goesto ground or logic 0). In some examples the voltages may not go all theway to supply or ground, e.g., because of other FETS and/or othercomponents that are in the circuit, etc. When nodes OUTM 209 and OUTP211 reach supply/ground (or near supply/ground), the transient currentshould become zero irrespective of CLK 201 being “low” or “high”.However, because either MN0 229 or MN1 231 would be in an ON state(depending on the output), current can flow from MP4 202 to MP0 203 orMP1 205 and to MN0 229 or MN1 231 until CLK 201 becomes “high”.

FIG. 2B is a graph 270 of the supply-to-ground or “through current” withrespect to the CLK 201, and the voltage at the output nodes OUTM 209 andOUTP 211. Referring to FIG. 2B, graph 270 shows that the through currentincreases when CLK 201 is low and one of outputs OUTP 211 and OUTM 209is low. In FIG. 2B, trace 271 corresponds to the through current, trace273 corresponds to CLK 201, and traces 275 and 277 correspond to outputsOUTM 209 and OUTP 211, respectively.

FIG. 3 shows an example differential amplifier and latch circuit 300that includes a supply-to-ground current prevention signal generator(e.g., 350). FIG. 3 shows a differential amplifier 300 that includestransistors 302, 303, 305, inputs INP 304 and INM 306 and outputs OUTM309 and OUTP 311. The differential amplifier 301 is coupled to a latch312 that includes transistors 313, 315, 329 and 331. In some examples,circuitry 350 to generate a signal to prevent supply-to-ground currentis coupled to the differential amplifier and the latch 312.

Referring to FIG. 3, the differential amplifier inputs INP 304 and INM306 are received by the transistors MP0 303 and MP1 305. In someexamples, when the CLK 332 goes “low,” the PMOS transistors (MP0 303,MP1 305, MP2 313, MP3 315) are switched on. In response, a differentialvoltage is developed at nodes OUTM 309 and OUTP 311. Moreover, by virtueof the operation of the latch 312, the nodes OUTM 309 and OUTP 311 go toeither supply or ground (e.g., the voltage at one of the nodes goes tosupply and the other of the nodes goes to ground). When nodes OUTM 309and OUTP 311 reach supply/ground, the transient current should becomezero irrespective of CLK 332 being “low” or “high.” However, becauseeither MN0 329 or MN1 331 would be in an ON state (depending on theoutput), current can flow from MP4 302 to MP0 303 or MP1 305 and to MN0329 or MN1 331 until CLK 332 becomes “high.” In examples, because thesupply-to-ground current prevention signal generator 350 acts togenerate a high clock signal when CLK 332 is still low, the path for thesupply-to-ground current is cut, and current flow from MP4 302 to MP0303 or MP1 305 and to MN0 329 or MN1 331 is prevented.

In operation, supply-to-ground current prevention signal generator 350:(1) determines if a decision is made at OUTM 309 and OUTP 311, and (2)based on the decision breaks the path between supply and ground when CLK332 is still “low”. In some examples, the operation of thesupply-to-ground current prevention signal generator 350 does not alterthe operation of the circuit when CLK 302 is “high.”

FIG. 4 shows example implementation of the supply-to-ground currentprevention signal generator 350 shown in FIG. 3. FIG. 4 shows inaddition to the components shown in FIG. 3, an example supply-to-groundcurrent prevention signal generator 350 that includes NAND gate 401, NORgate 403 and inverter 405. Referring to FIG. 4, when differentialoutputs OUTM 309 and OUTP 311 go to supply and ground (logic 1 and 0 andvice versa) logic levels 1 and 0 are placed on the respective inputs tothe NOR gate 403 and the low clock signal is provided as the input toinverter 405. The differential inputs to the NOR gate 403 cause theoutput of the NOR gate 403 to be 0. The 0 placed on the input to theinverter 405 causes the output of the inverter 405 to be 1. The outputof the NOR gate 403, logic level 0, and the output of the inverter 405,logic level 1, are placed on the respective inputs to NAND gate 401. Assuch, the output of the NAND gate 401 is logic level 1, which causestransistor MP4 302 to turn off. In some examples, when the transistorMP4 302 turns OFF, the path for the supply-to-ground current is cut(e.g., MP4 302 does not conduct current), and current flow from MP4 302to MP0 303 or MP1 305 and to MN0 329 or MN1 331 is prevented.

FIG. 5 shows a second example circuit placement of a supply-to-groundcurrent prevention signal generator 550. In the FIG. 5 example, thesupply-to-ground current prevention signal generator 550 is connected tothe drain of a transistor 552 that is coupled to the supply voltage. Inaddition, an output 554 of the supply-to-ground current preventionsignal generator 550 is coupled to the sources of transistors of thedifferential amplifier of the associated comparator (MP0 and MP1 inFIGS. 6 and 7). In contrast, in FIG. 3, the supply-to-ground currentprevention signal generator 350 is connected to the gate of a transistorthat is coupled to the supply voltage.

FIG. 6 shows an example implementation of the supply-to-ground currentprevention signal generator 550 shown in FIG. 5. FIG. 6 shows inaddition to the components shown in FIG. 3, supply-to-ground currentprevention signal generator 650 that includes a circuit that consists oftwo PMOS transistors 601 and 603 that are added in series withtransistors MP0 303 and MP1 305. Referring to FIG. 6, when differentialoutputs OUTM 309 and OUTP 311 go to supply and ground (logic 1 and 0 andvice versa) logic levels 1 and 0 are placed on the respective inputs toPMOS transistors 601 and 603 that are added in series with transistorsMP0 303 and MP1 305. The transistors 601 and 603 prevent current fromflowing through each of the branches connected to the source oftransistor MP4 302. The gates of transistors 601 and 603 are coupled toQ and QZ, which are the same as the outputs of the comparator, becauseeach of Q and QZ are generated from a comparator output through twoinverters 317, 319. In this manner, the gate of transistor 601 or thegate of transistor 603 is transitioned to a high voltage in order toturn the transistor off in response to a comparator decision. As such,the current that flows through transistor MP4 302 is prevented fromflowing through each of the branches connected to the source oftransistor MP4 302. In some examples, when the current from transistorMP4 302 is prevented from flowing through each of the branches, the pathfor the supply-to-ground current is cut, and current flow from MP4 302to MP0 303 or MP1 305 and to MN0 329 or MN1 331 is prevented.

FIG. 7 shows a second example implementation of the system to generate aclock shown in FIG. 5. FIG. 7 shows in addition to the components shownin FIG. 3, supply-to-ground current prevention signal generator 550 thatincludes OR gate 701 coupled to the gate of transistor MP6 703.Referring to FIG. 7, when differential outputs OUTM 309 and OUTP 311 goto supply and ground (logic 1 and 0 and vice versa) logic levels 1 and 0are placed on the respective inputs to OR gate 701. As such, the currentthat flows through transistor MP4 301 is prevented from flowing througheach of the branches connected to the source of transistor MP4. In someexamples, when the current from transistor MP4 302 is prevented fromflowing through each of the branches, the path for the supply-to-groundcurrent is cut (the “through current”), and current flow from MP4 302 toMP0 303 or MP1 305 and to MN0 329 or MN1 331 is prevented.

FIG. 8 shows components of a supply-to-ground current prevention signalgenerator 350. FIG. 8 shows state determining circuitry 801, currentflow prevention signal generating circuitry 803 and current flowprevention signal supplying circuitry 805.

Referring to FIG. 8, state determining circuitry 801 determines if thevoltage at the first and the second output nodes of a comparator (e.g.,comprising an amplifier and a latch) has reached supply or ground. Insome examples, state determining circuitry 801 includes circuitry thatis coupled to first and second output nodes of the comparator.

Current flow prevention signal generating circuitry 803 responsive to adetermination of the voltages at the first and the second output nodes,generates a signal that causes the prevention of the flow ofsupply-to-ground current through the comparator. In some examples,current flow prevention signal generating circuitry 803 generates amodified clock that turns off a transistor that couples the comparatorto the supply voltage to ensure that the flow of supply-to-groundcurrent through the comparator is prevented. In other examples, supplyto ground current prevention signal generator 350 generates a signalthat turns off a transistor coupled to the transistor that couples thecomparator to the supply voltage to ensure that the flow ofsupply-to-ground current through the comparator is prevented.

Current flow prevention signal supplying circuitry 805 supplies avoltage level to one or more transistors of the comparator that turnsoff one or more transistors to prevent the flow of supply-to-groundcurrent through the comparator. In some examples, the current flowprevention signal supplying circuitry 805 supplies the voltage level tothe one or more transistors responsive to a determination that thevoltage at the output nodes of the comparator has reached supply andground respectively.

In some examples, supply-to-ground current prevention signal generator350 includes a NAND gate and a NOR gate. In some examples,supply-to-ground current prevention signal generator 350 includes aplurality of p-type MOSFETS. In some examples, supply-to-ground currentprevention signal generator 350 can include an OR gate coupled to afirst transistor that is coupled to a second transistor that is coupledto the supply voltage that is coupled to the comparator. In someexamples, the comparator includes a differential amplifier and a latch.

While an example manner of implementing the supply-to-ground currentprevention signal generator 350 of FIG. 3 is illustrated in FIG. 8, oneor more of the elements, processes and/or devices illustrated in FIG. 8may be combined, divided, re-arranged, omitted, eliminated and/orimplemented in any other way. Further, the example state determiningcircuitry 801, current flow prevention signal generating circuitry 803and current flow prevention signal supplying circuitry 805 and/or, moregenerally, the example supply-to-ground current prevention signalgenerator 350 of FIG. 3 may be implemented by hardware, software,firmware and/or any combination of hardware, software and/or firmware.Thus, for example, any of the example state determining circuitry 801,current flow prevention signal generating circuitry 803 and current flowprevention signal supplying circuitry 805 and/or, more generally, theexample supply-to-ground current prevention signal generator 350 couldbe implemented by one or more analog or digital circuit(s), logiccircuits, programmable processor(s), application specific integratedcircuit(s) (ASIC(s)), programmable logic device(s) (PLD(s)) and/or fieldprogrammable logic device(s) (FPLD(s)). When reading any of theapparatus or system claims of this patent to cover a purely softwareand/or firmware implementation, at least one of the example, statedetermining circuitry 801, current flow prevention signal generatingcircuitry 803 and current flow prevention signal supplying circuitry 805is/are hereby expressly defined to include a tangible computer readablestorage device or storage disk such as a memory, a digital versatiledisk (DVD), a compact disk (CD), a Blu-ray disk, etc. storing thesoftware and/or firmware. Further still, the example supply-to-groundcurrent prevention signal generator 350 of FIG. 3 may include one ormore elements, processes and/or devices in addition to, or instead of,those illustrated in FIG. 4, and/or may include more than one of any orall of the illustrated elements, processes and devices.

A flowchart representative of example machine readable instructions forimplementing the supply-to-ground current prevention signal generator350 of FIG. 3 is shown in FIG. 9. In this example, the machine readableinstructions comprise a program for execution by a processor such as theprocessor 1012 shown in the example processor platform 1000 discussedbelow in connection with FIG. 10. The program may be embodied insoftware stored on a tangible computer readable storage medium such as aCD-ROM, a floppy disk, a hard drive, a digital versatile disk (DVD), aBlu-ray disk, or a memory associated with the processor 1012, but theentire program and/or parts thereof could alternatively be executed by adevice other than the processor 1012 and/or embodied in firmware ordedicated hardware. Further, although the example program is describedwith reference to the flowchart illustrated in FIG. 9, many othermethods of implementing the example supply-to-ground current preventionsignal generator 350 may alternatively be used. For example, the orderof execution of the blocks may be changed, and/or some of the blocksdescribed may be changed, eliminated, or combined.

As mentioned above, the example processes of FIG. 9 may be implementedusing coded instructions (e.g., computer and/or machine readableinstructions) stored on a tangible computer readable storage medium suchas a hard disk drive, a flash memory, a read-only memory (ROM), acompact disk (CD), a digital versatile disk (DVD), a cache, arandom-access memory (RAM) and/or any other storage device or storagedisk in which information is stored for any duration (e.g., for extendedtime periods, permanently, for brief instances, for temporarilybuffering, and/or for caching of the information). As used herein, theterm tangible computer readable storage medium is expressly defined toinclude any type of computer readable storage device and/or storage diskand to exclude propagating signals and to exclude transmission media. Asused herein, “tangible computer readable storage medium” and “tangiblemachine readable storage medium” are used interchangeably. Additionallyor alternatively, the example processes of FIG. 9 may be implementedusing coded instructions (e.g., computer and/or machine readableinstructions) stored on a non-transitory computer and/or machinereadable medium such as a hard disk drive, a flash memory, a read-onlymemory, a compact disk, a digital versatile disk, a cache, arandom-access memory and/or any other storage device or storage disk inwhich information is stored for any duration (e.g., for extended timeperiods, permanently, for brief instances, for temporarily buffering,and/or for caching of the information). As used herein, the termnon-transitory computer readable medium is expressly defined to includeany type of computer readable storage device and/or storage disk and toexclude propagating signals and to exclude transmission media. As usedherein, when the phrase “at least” is used as the transition term in apreamble of a claim, it is open-ended in the same manner as the term“comprising” is open ended.

FIG. 9 is a flowchart of a method to generate a supply-to-ground currentprevention signal.

Referring to FIG. 9, state determining circuitry 801 determines if thevoltage at the first and the second output nodes of a comparator (e.g.,comprising an amplifier and a latch) has reached supply or ground (block901). In some examples, the voltages at the first and second outputnodes may be initially set to a low voltage (e.g., a ground voltage)during a reset phase, and the state determining circuitry 801 may,during a comparison phase, detect when at least one of the voltages atthe first and second output nodes transitions from a low voltage to ahigh voltage. In some examples, state determining circuitry 801 includescircuitry that is coupled to first and second output nodes of thecomparator.

Current flow prevention signal generating circuitry 803 responsive to adetermination of the voltages at the first and the second output nodes(e.g., a determination that at least one of the voltages at the firstand second output nodes transitions from a low voltage to a highvoltage), generates a signal that causes the prevention of the flow ofsupply-to-ground current through the comparator (block 903). In someexamples, current flow prevention signal generating circuitry 803generates a modified clock that turns off a transistor that couples thecomparator to the supply voltage to ensure that the flow ofsupply-to-ground current through the comparator is prevented. In otherexamples, current flow prevention signal generating circuitry 803generates a signal that turns off a transistor coupled to the transistorthat couples the comparator to the supply voltage to ensure that theflow of supply-to-ground current through the comparator is prevented.

Current flow prevention signal supplying circuitry 805 supplies avoltage level to one or more transistors of the comparator that turnsoff one or more transistors to prevent the flow of supply-to-groundcurrent through the comparator (block 905). In some examples, thecurrent flow prevention signal supplying circuitry 805 supplies thevoltage level to the one or more transistors responsive to adetermination that the voltage at the output nodes of the comparator hasreached supply and ground respectively.

FIG. 10 is a block diagram of an example processor platform 1000 capableof executing the instructions of FIG. 9 to implement thesupply-to-ground current prevention signal generator 350 of FIG. 3. Theprocessor platform 1000 can be, for example, a server, a personalcomputer, a mobile device (e.g., a cell phone, a smart phone, a tabletsuch as an iPad™), a personal digital assistant (PDA), an Internetappliance, a DVD player, a CD player, a digital video recorder, aBlu-ray player, a gaming console, a personal video recorder, a set topbox, or any other type of computing device.

The processor platform 1000 of the illustrated example includes aprocessor 1012. The processor 1012 of the illustrated example ishardware. For example, the processor 1012 can be implemented by one ormore integrated circuits, logic circuits, microprocessors or controllersfrom any desired family or manufacturer.

The processor 1012 of the illustrated example includes a local memory1013 (e.g., a cache). The processor 1012 of the illustrated example isin communication with a main memory including a volatile memory 1014 anda non-volatile memory 1016 via a bus 1018. The volatile memory 1014 maybe implemented by Synchronous Dynamic Random Access Memory (SDRAM),Dynamic Random Access Memory (DRAM), RAMBUS Dynamic Random Access Memory(RDRAM) and/or any other type of random access memory device. Thenon-volatile memory 1016 may be implemented by flash memory and/or anyother desired type of memory device. Access to the main memory 1014,1016 is controlled by a memory controller.

The processor platform 1000 of the illustrated example also includes aninterface circuit 1020. The interface circuit 1020 may be implemented byany type of interface standard, such as an Ethernet interface, auniversal serial bus (USB), and/or a PCI express interface.

In the illustrated example, one or more input devices 1022 are connectedto the interface circuit 1020. The input device(s) 1022 permit(s) a userto enter data and commands into the processor 1012. The input device(s)can be implemented by, for example, an audio sensor, a microphone, acamera (still or video), a keyboard, a button, a mouse, a touchscreen, atrack-pad, a trackball, isopoint and/or a voice recognition system.

One or more output devices 1024 are also connected to the interfacecircuit 1020 of the illustrated example. The output devices 1024 can beimplemented, for example, by display devices (e.g., a light emittingdiode (LED), an organic light emitting diode (OLED), a liquid crystaldisplay, a cathode ray tube display (CRT), a touchscreen, a tactileoutput device, a printer and/or speakers). The interface circuit 1020 ofthe illustrated example, thus, typically includes a graphics drivercard, a graphics driver chip or a graphics driver processor.

The interface circuit 1020 of the illustrated example also includes acommunication device such as a transmitter, a receiver, a transceiver, amodem and/or network interface card to facilitate exchange of data withexternal machines (e.g., computing devices of any kind) via a network1026 (e.g., an Ethernet connection, a digital subscriber line (DSL), atelephone line, coaxial cable, a cellular telephone system, etc.).

The processor platform 1000 of the illustrated example also includes oneor more mass storage devices 1028 for storing software and/or data.Examples of such mass storage devices 1028 include floppy disk drives,hard drive disks, compact disk drives, Blu-ray disk drives, RAIDsystems, and digital versatile disk (DVD) drives.

The coded instructions 1032 of FIG. 9 may be stored in the mass storagedevice 1028, in the volatile memory 1014, in the non-volatile memory1016, and/or on a removable tangible computer readable storage mediumsuch as a CD or DVD.

Although certain example methods, apparatus and articles of manufacturehave been disclosed herein, the scope of coverage of this patent is notlimited thereto. On the contrary, this patent covers all methods,apparatus and articles of manufacture fairly falling within the scope ofthe claims of this patent.

What is claimed is:
 1. An apparatus to prevent supply-to-ground currentin a comparator, the apparatus comprising: circuitry to determine iffirst and second output nodes of the comparator have respectivelyreached first and second logic levels; circuitry responsive to adetermination that voltages at the first and second output nodes of thecomparator have respectively reached the first and second logic levels,to generate a signal; and circuitry to supply the signal to a firsttransistor, the signal to turn off the transistor and prevent the flowof supply-to-ground current through the comparator.
 2. The apparatus ofclaim 1, wherein the circuitry responsive to a determination that thevoltage at the first or second output nodes of the comparator hasreached a first state includes a plurality of logic gates.
 3. Theapparatus of claim 1, wherein the circuitry responsive to adetermination that the voltage at the first or second output nodes ofthe comparator has reached a first state includes a NAND gate and a NORgate.
 4. The apparatus of claim 1, wherein the circuitry responsive to adetermination that the voltage at the one or more of the first or secondoutput nodes of the comparator has reached a first state includes aplurality of n-type MOSFETS coupled to the first transistor.
 5. Theapparatus of claim 1, wherein the circuitry responsive to adetermination that the voltage at the first or second output nodes ofthe comparator has reached a first state includes a NOR gate coupled toa second transistor that is coupled to the first transistor.
 6. Theapparatus of claim 1, wherein the comparator includes a differentialamplifier and a latch.
 7. The apparatus of claim 6, wherein thecircuitry responsive to a determination that the voltage at the first orsecond output nodes of the comparator has reached a first state iscoupled between the comparator and the latch.
 8. An apparatus togenerate a signal to prevent supply-to-ground current in a comparator,the apparatus comprising: circuitry to determine if a voltage at anoutput node of the comparator has reached a first state; circuitryresponsive to a determination that the voltage at the one or more outputnodes of the comparator has reached the first state, to generate avoltage of a second state to turn off a transistor that is coupled to asupply voltage of the comparator; and circuitry to supply the voltage ofthe second state to the transistor that is coupled to the supply voltageto the comparator.
 9. The apparatus of claim 8, wherein the circuitryresponsive to a determination that the voltage at the output node of thecomparator has reached a first state includes a plurality of logicgates.
 10. The apparatus of claim 8, wherein the circuitry responsive toa determination that the voltage at the output node of the comparatorhas reached a first state includes a NAND gate and a NOR gate.
 11. Theapparatus of claim 8, wherein the circuitry responsive to adetermination that the voltage at the output node of the comparator hasreached a first state includes a plurality of n-type MOSFETS coupled tothe transistor.
 12. The apparatus of claim 8, wherein the circuitryresponsive to a determination that the voltage at the output node of thecomparator has reached a first state includes an OR gate coupled to asecond transistor that is coupled to the transistor.
 13. The apparatusof claim 8, wherein the comparator includes a differential amplifiercoupled to a latch.
 14. The apparatus of claim 8, wherein the circuitryresponsive to a determination that the voltage at the output node of thecomparator has reached a first state is coupled between the comparatorand a latch.
 15. An method to generate a signal to preventsupply-to-ground current in a comparator, the method comprising:determining if a voltage at an output node of the comparator has reacheda first state; responsive to a determination that the voltage at the oneor more output nodes of the comparator has reached the first state,generating a voltage of a second state to turn off a transistor that iscoupled to a supply voltage of the comparator; and supplying the voltageof the second state to the transistor that is coupled to the supplyvoltage to the comparator.
 16. The method of claim 15, wherein circuitryresponsive to a determination that the voltage at the output node of thecomparator has reached a first state includes a plurality of logicgates.
 17. The method of claim 15, wherein circuitry responsive to adetermination that the voltage at the output node of the comparator hasreached a first state includes a NAND gate and a NOR gate.
 18. Themethod of claim 15, wherein circuitry responsive to a determination thatthe voltage at the output node of the comparator has reached a firststate includes a plurality of n-type MOSFETS coupled to the transistor.19. The method of claim 15, wherein circuitry responsive to adetermination that the voltage at the output node of the comparator hasreached a first state includes an OR gate coupled to a second transistorthat is coupled to the transistor.
 20. The method of claim 15, whereinthe comparator includes a differential amplifier and a latch.